There is a major effort in the communication industry to add broadband video services to the telephone network. For technical and economic reasons, the transmitted video will most likely be in digital form, requiring a data rate of over 100 Mb/s. Since this high information rate is ten thousand-fold greater than that of digitized voice, it will require techniques that emphasize high-speed performance.
Although optical switching has great switching potential, it is not yet ready for deployment. Consequently, electronic switching will be utilized in the first introduction of broadband video. The industry is focusing on CMOS space switching as the most straightforward broadband switching technique due to its high speed, high density, and low power dissipation. However, conventional broadband space switches reported have been limited to predominantly 16.times.16 arrays. As the size of the arrays is increased, the switching speed decreases due to a geometric increase in crosspoints and associated parasitic elements.
The source of speed limitations in conventional space switch arrays is illustrated with the equivalent circuit of FIG. 1. The array contains M inputs, each of which can be connected to N outputs by closing the switch at the intersection of an input/output line. The switches have associated stray capacitances that cause speed degradation. Therefore the speed decreases as the size of the array is increased. For example, by closing switch S11, input 1 is connected to output 1. Even though inputs 2 to M are not connected, they contribute to the stray capacitance of column 1. Similarly, even though columns 2 to N are not connected, they contribute to the stray capacitance of row 1. It can be seen that input line 1 must charge (N-1)+(M-1) capacitors. The finite resistance in series with line 1 and column 1 forms an RC time constant that limits the speed of operation. As the array size is increased, this stray capacitance also increases and the speed continues to decrease.
The stray capacitance of the horizontal rows can be overcome by providing sufficient drive to the input lines. The most detrimental effect is caused by connections to the vertical lines. This is due to the fact that in reality each of the switches at the crosspoints is an active circuit that must drive the vertical line and its associated capacitive loading. It does not help to make the active switch element larger so it can drive more capacitance because the stray capacitance increases in almost direct proportion to the size of the active switch.